Diesel engines are well-known to provide significant advantages in fuel efficiency and continue to be the subject of intensive development and further improvements. However, controlling the emission of oxides of nitrogen (NOx) has proven quite challenging, since control techniques tend to increase the emissions of other pollutants or decrease fuel economy. Proposed regulations provide further challenges to manufacturers to achieve good fuel economy and reduce NOx.
In the case of diesel engines, NOx reduction has typically been achieved through the alteration of engine operating parameters that impact combustion. Among these engine operating parameters, injection timing is one of the most influential factors in controlling NOx emissions. By retarding the injection timing, significant reductions in NOx emissions can be achieved. Injection rate also affects NOx emissions, with increased rates resulting in increased NOx emissions. Likewise, any change that increases combustion temperature (increased compression ratio, fuel-air ratio, etc.) increases NOx emissions. Exhaust gas recirculation (EGR) is a method used to reduce combustion temperatures and therefore NOx emissions. Unfortunately, the use of exhaust gas recirculation increases particulate emissions in diesel engines, limiting the practical level to which it can be used to about 15%.
Three-way catalysts are widely used in gasoline engines to reduce NOx emissions. To efficiently use such catalysts, the engine must be operated at or near stoichiometry. Since diesel engines are operated under oxygen rich conditions, even at full load, typical gasoline engine exhaust gas catalyst systems are ineffective for reducing NOx. Non-diesel, lean burn engines provide similar challenges in the reduction of NOx emissions, in that conventional three-way catalyst systems are ineffective. As used herein, the term lean burn is meant to include engines that can be operated with an oxygen concentration greater than the amount required for stoichiometric combustion of a hydrocarbon fuel, (e.g., at least 1% by weight excess oxygen). Such engines include all combustors which combust hydrocarbon fuels to provide heat, e.g., for direct or indirect conversion to mechanical or electrical energy, internal combustion engines of the Otto, Diesel and turbine types, as well as burners and furnaces.
Efforts to reduce NOx in diesel and other lean burn engines have included a variety of catalytic and non-catalytic techniques. Techniques employing a catalytic reduction method essentially comprise passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Non-catalytic techniques include selective non-catalytic reduction (SNCR) systems. Catalytic techniques have included the use of quaternary catalyst reduction systems, hydrocarbon selective catalyst reduction (SCR) systems and urea SCR systems. Since quaternary catalysts have a low nitric oxide reduction efficiency (about 10%), they are not particularly effective. Hydrocarbon SCR systems are known to have a nitric oxide reduction efficiency of about 35%, with a penalty to fuel efficiency of about 3%. Urea SCR systems have a nitric oxide reduction efficiency that can exceed 65%. In view thereof, urea SCR systems have received considerable interest by industry.
SCR systems have been available for years for reducing NOx emissions from fixed-base sources. SCR systems depend on the use of ammonia, which has safety problems associated with its storage and transport. Urea is safer, but has not been practical for many SCR applications, particularly mobile NOx sources, due to the difficulty in converting it from a solid or an aqueous form to its active gaseous species.
NOx reducing catalysts have been developed which are effective over the operating range of the engine. Despite the infrastructure concerns relating to the use of urea in a mobile application, as well as the potentially dangerous risks of ammonia break-through (slip), ammonia SCR systems are becoming the favored choice for mobile applications to meet more stringent NOx emissions. This is due to the high NOx conversion percentages, mentioned above, that are possible with ammonia, coupled with the ability to optimize the combustion process for maximum power output with minimum fuel consumption.
Much work has been undertaken to reduce NOx emissions in diesel engines. As disclosed in U.S. Pat. No. 4,188,364, in order to enable ammonia to react with NOx, at typical combusted gas stream temperatures of a diesel or spark-ignited engine, it is known to mix gaseous ammonia into the combusted gas stream, in proportion to the amount of NOx contained in the combusted gas stream, and then route the gaseous mixture to a catalytic reactor.
With regard to SCR control systems, U.S. Pat. No. 4,403,473 proposes an ammonia/fuel ratio control system for reducing nitrogen oxide emissions wherein ammonia is metered to the combusted gas stream in a pre-selected proportion to the fuel mass flow rate and in response to the sensed temperature of the combusted gas stream in the reactor being within a pre-selected range.
U.S. Pat. No. 5,116,579 measures the humidity of intake air and one or more operating parameters of engine power, intake air temperature, fuel consumption and exhaust gas temperature to set an ammonia ratio control valve. The molar ratio of ammonia to NOx is set at less than one to minimize ammonia slip.
U.S. Pat. No. 5,522,218 proposes a combustion exhaust purification system and method for use in relatively large diesel engines. A computer controlled injector intermittently injects an amount of NOx reducing fluid into the exhaust passageway from the engine. The amount of NOx reducing fluid said to be introduced corresponds to an amount that will achieve improved NOx reduction rates for the given engine operating condition and exhaust temperature. A computer periodically senses the engine operating condition and the exhaust temperature, and calculates the injection amount.
With respect to control systems, Japan Publication No. JP-A 55093917 proposes techniques for the detection of exhaust gas flow rate, nitrogen oxide concentration in the exhaust gas, exhaust gas temperature, along with the temperature of the denitration deNOx catalyst. Through the use of those data and in accordance with the nitrogen oxide rate, a rate of introduction into the exhaust gas is calculated and performed that takes into account the catalytic activity of the catalyst, which is dependent on the catalyst temperature. It has been reported by others, however, that such a method, especially during positive and negative sudden load changes, does not prevent an escape of reducing agent (slip) or nitrogen oxide.
U.S. Pat. No. 5,628,186 proposes a method and apparatus for the controlled introduction of a reducing agent into a nitrogen oxide-containing exhaust gas of an internal combustion engine having a catalytic converter for reducing nitrogen oxide. The method includes detecting at least one operation-relevant parameter of the exhaust gas of the catalytic converter and optionally of the engine to determine the nitrogen oxide rate. An intermediate value is determined for the reducing agent rate as a function of the nitrogen oxide rate. The intermediate value is reduced by a rate of the reducing agent desorbed by the catalytic converter or raised by a rate of the reducing agent adsorbed by the catalytic converter. An apparatus for performing the method includes a control unit. The control unit is intended to adjust a rate of the reducing agent introduced into the exhaust gas as a function of the parameters, while taking into account a rate of the reducing agent adsorbed by the catalytic converter or desorbed by the catalytic converter.
Notwithstanding the advances in hydrocarbon-, urea- and ammonia-based SCR systems, the reducing agent delivery and control systems developed to date have proven to be complicated and/or ineffective to control the SCR system under all engine operating conditions. This problem is particularly acute when the impact of transient NOx emissions on the SCR system is considered. As may be appreciated, due to the continual variance in engine speed and load, the quantities of nitrogen oxide generated by per unit time and the flow rates and temperatures of the exhaust gas are subject to major fluctuations.
While urea-based SCR systems possess many advantages over hydrocarbon- and ammonia-based systems, it is difficult to rapidly adjust the quantity of reducing agent introduced into the exhaust gas per unit of time during transient conditions. Another difficulty lies in the inability to promote good mixing of the reducing agent with the exhaust gas under all transient conditions. Failure to promote good mixing and rapidly adjust the quantity of reducing agent greatly impacts NOx reduction efficiencies and can result in ammonia slip. Ammonia slip represents a serious problem, since ammonia is poisonous, and even at a concentration of only about 5 ppm, it represents a considerable odor burden to humans. For that reason, an escape of ammonia must be avoided.
Therefore, there is a need for a safe, economical and effective reducing agent delivery system to address the problems associated with SCR systems, particularly for mobile diesel and other lean burn engines.